Alpha 374A or 76 series power switch

Discussion in 'Amateur Radio Amplifiers' started by AD0AC, Sep 8, 2019.

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  1. N8ZI

    N8ZI XML Subscriber QRZ Page

    Do you have the plans?
     
  2. W1QJ

    W1QJ Ham Member QRZ Page

    Go to K6xx web page. He has a whole discussion on a grid trip circuit using a 4N35. I simplified his idea and I used a latching relay circuit that was tripped by the 4n35 and latched the relay which opened the key line. A small momentary push button switch on front panel resets it. The 4n35 across the grid shunt resistor trips at a perfect voltage that relates to a decent trip point for the grid. Most effective and cheap cheap.
     
    AD0AC likes this.
  3. AD0AC

    AD0AC Ham Member QRZ Page

    Well, things seems to be working okay now. Got the lamp covers sorted out and I replaced the RCA input jack with a BNC, so my exciter plays well with it now. I also had a couple of loose screws on two of the HV caps, so I tightened them down and now get the full 2.2 kV with the SSB switch. I left it on for a couple hours this evening to let everything settle and I'm seeing about 850-950 watts out in bandpass mode with 100 watts in from the exciter in CW on 20 meters. I don't think the low input mod was ever done on this one. I'm going to try it on the air tomorrow and see how it plays, and try some other bands too.

    Anyone know how to lube the blower on this things? It's pretty rough.
     
  4. N8ZI

    N8ZI XML Subscriber QRZ Page

    Lubing the blower motor is easy on my 374A. A screw holds a plate over the oil fill hole on the back of the blower. There is a fairly large hole through the back panel for this service.
    The blower bushing is a porous oil impregnated type.
    Stand the amp face down, rear panel up, pull the screw and pop the cover off, filll with light oil, 3n1, sewing machine oil. Allow time for the oil to soak into the bushing and fill the reservoir again.
    The cover has an O ring seal so the oil doesn't leak out.
    Put the cover and screw back on and it is good to go for several more years.
     

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